Reworkable b-stageable adhesive and use in waferlevel underfill

ABSTRACT

A reworkable thermoset epoxy-containing material that allows for a reworkable assembly such as a reworkable waferlevel underfilled miocroelectronic package. A method for using the reworkable thermoset material in the formation of a microelectronic package using this material.

FIELD OF THE INVENTION

This invention relates to reworkable thermoset epoxy-containing materialthat allows for a reworkable assembly, for example a reworkablewaferlevel underfilled microelectronic package. The invention alsorelates to a method using the reworkable thermoset matrial in theformation of a microelectronic package using this material, and otherstructures containing the reworkable thermoset epoxy-containingmaterial.

PRIOR ART

Flip Chip technology has grown rapidly in recent years since it allowsto decrease chip footprint while simultaneously increasing the number ofpossible I/O's. This is because flip chip technology takes advantage ofthe chip area for I/O's instead of just the chip periphery as in wirebonded chips. Various methods exist for solder bumping wafers, such asevaporation, plating, solder paste screening and more recently,injection molded solder, (IMS).

Notwithstanding how the wafers are bumped, the bumped wafers typicallyare diced in the next step into separate chips. For direct chip attach,(DCA), silicon chips are bonded directly to a laminate substrate. DCA isa rapidly growing packaging technology since it requires the smallestamount of area on the laminate, has the smallest height, and is lighterthan other packages. Additionally, flip chip applications provide betterelectrical characteristics and better cooling than wire bond packagedchips. However, since there is a significant mismatch of the coefficientof thermal expansion (CTE) between the chip silicon and the laminatematerial, DCA bonded chips must be underfilled with an adhesive supportmaterial; underfilling greatly increases the fatigue life of the solderbumps.

However, several manufacturing problems are associated withunderfilling.

First, the process adds to the time required to bond each chip. This isdue to the fact that the dispensed underfill must flow into the gapbetween the chip and substrate driven by capillary forces alone. Thedispensing operation requires from 30 to 120 seconds depending on thedie geometry, bump pattern, standoff, and material characteristics. Uponcompletion of the underfill operation, batch curing is done requiring anadditional amount of time.

Secondly, the distribution of the underfill may be incomplete since theconventional underfill process relies on capillary action to pull theliquid underfill completely under the chip. Since it is difficult todetermine underfill uniformity under the chip, partial fills occur whichmay lead to excess mechanical stress on affected bumps. To avoid thisproblem, many flip chip packages are 100% inspected by scanning acousticmicroscopy, an expensive and slow process.

Thirdly, underfilling adds cost since extra tooling, 0 line space andoperator personnel are required.

In order to eliminate these shortcomings several processes have beenproposed such as the “no-flow process” and the “waferlevel underfillprocess.” While the no-flow process requires that the underfill materialis unfilled, wafer-level underfill processes can be exercised usingfilled materials.

Typical process flows for no-flow and waferlevel underfill are shown inFIG. 1 a and 1 b, respectively.

In no-flow underfill (FIG. 1 a), the main steps involve dispensing anunderfill on a substrate; placing and aligning the chip to the substratepattern; softening the underfill on the substrate by heating; pushingthe aligned bumps through the underfill so that the pads are contacted;and reflowing the solder to make the electrical joint between the chipand substrate. The underfill may or may not require a cure step. Thismethod still requires dispense tooling and individual application ofunderfill for each chip site. Filled materials have been found not towork in no-flow applications due to the inclusion of particles in thesolder joints which lead to electrically inferior and mechanically weakjoints.

In conventional waferlevel underfill (FIG. 1 b), a filled material isapplied to a bumped wafer so that the bumps remain free of the underfillmaterial. This is done mostly by screening the material, or by using amolding process. Spin application followed by laser or plasma etching ofthe material covering the solder bumps have also been proposed. Theseapplication methods are deemed necessary to avoid inclusion of fillerparticles in the solder joint which is thought to increase electricalresistance and decrease mechanical integrity.

In the next step the underfill on the wafer is “b-staged.” In the art, ab-stage resin is a thermosetting resin reacted to a stage where it ishard at room temperature and essentially solvent free but still flowswhen heated above its b-stage T_(g). It is a preferred stage for a resinwhen it is being molded. In the step where the wafer is b-staged, thesolvent is removed and the crosslink reaction is advanced tosignificantly below the gel point with the goal to render the b-stagedunderfill surface tack free and to impart desired b-stage T_(g) and flowproperties.

In the next step the wafer is diced into underfilled chips. Duringdicing operation the underfill needs to stay below the T_(g) so as notto foul or “gum up” the dicing saw. Further the b-staged underfill needsto adhere to the passivation layer of the chip during dicing. The dicedchips now can be stored.

In the prior art, epoxy based materials are most widely used asunderfills. Epoxy resins are of great importance for a number of diverseapplications including coatings, adhesives, structural materials,electrical insulation, encapsulants etc. Epoxy formulations haveoutstanding properties after curing, including, but not limited to,toughness, adhesion and solvent resistance.

Another attribute of epoxy thermosets is their intractability aftercuring. This intractability is only another aspect of the chemistry ofthermosets, which makes use of a curing reaction to convert lowmolecular weight precursors to a network polymer of essentially infinitemolecular weight. This same property of intractability of thermosets,however, is a liability since it prohibits rework or at least makes itvery difficult. If expensive chips or substrates are being used, theinability to rework is not acceptable since one defective part wouldrender the whole assembly useless.

Another concern is the longevity of thermosets in the environment.Already many manufacturers are taking responsibility for disposal orrecycling of their products, and others are being required to do so bygovernment regulation. As part of this trend, the concept of design fordisassembly is one that is gaining in favor. Intractable thermosets arenot compatible with this concept, whether they are used as structuralcomponents, adhesives, or encapsulants. If, however, the thermosetitself is designed for controlled disassembly on the molecular scale, itis possible that many advantages of thermosets can be retained withoutthe disadvantage of intractability.

In underfills, thermosets act as adhesives effectively gluing componentsto a substrate and encapsulating the electrical connections between flipchip and said substrate. If a substrate holds more than one underfilledcomponent as in high performance Multi Chip Modules (MCMs) andFlip-chip-on-Board (FCOB) applications, the inability to disassemble or“rework” a defective underfilled component can become very expensivesince the whole package will become useless. A rework process for suchattached chips is highly desirable.

Thermoset adhesive connections can be broken by heating an assemblyabove the glass transition and applying force. While this is not adesirable process, it is a possibility. However, the site of theoriginal, adhering, defective component, such as a flip chip, needs tobe cleaned to receive a new, working component. This site cleanoperation remains still the major roadblock to successful rework ofthermoset underfilled chips.

Buchwalter, et al. developed diepoxides with acid-cleavable acetalgroups. This approach works well with respect to the clean-up aspect ofresidue removal and preparation of the site in the reworkable process.However, the materials proposed in Buchwalter, et al.'s prior artdisclosures are of the conventional underfill type, which carry with itthe problems outlined above.

There are also reports on thermally reworkable underfill. This approachinvolves high temperature and harsh mechanical processes to clean thesite. Another approach is to use thermoplastic as an encapsulant but thethermoplastic material requires high temperature to melt and longexposure to solvent in order to dissolve the polymer. The use ofreworkable epoxies and epoxy formulations is of course not restricted tounderfill applications but they can be used in all applications whereepoxies and their formulations are used.

Some other pertinent disclosures relating to diepoxides and their usesin such applications as waferlevel underfill are found the followingreferences, the contents of which are incorporated by reference herein:U.S. Pat. No. 5,512,613 to Ali Afzali-Ardakani, S. L. Buchwalter, etal., “Cleavable Diepoxide for Removable Epoxy Compositions”; U.S. Pat.No. 5,560,934 to Buchwalter, et al., “Cleavable Diepoxide for RemovableEpoxy Compositions”; U.S. Pat. No. 6,258,899 to S. L. Buchwalter, etal., “Cleavable Acetal-containing Diepoxide and Anhydride Curing Agentfor Removable Epoxy Compositions”; S. L. Buchwalter and L. L. Kosbar,“Cleavable Epoxy Resins: Design for Disassembly of Thermoset,” J. Polym.Sci., Part A: Polym Chem., Vol. 34, P. 249 (1996); S. L. Buchwalter, A.J. Call, et al., “Reworkable Epoxy Underfill for Flip-Chip Packaging,”First International Symposium on Advanced Packaging Materials, Process,Properties, and Interfaces, ISHM, February P. 7 (1995); J. Rudolph, K.Laxma Reddy, J. P. Chiang, and K. Barry Sharpless “Highly EfficientEpoxidation of Olefins Using Aqueous H₂O₂ and CatalyticMethyltrioxorhenium/Pyridine: Pyridine-Mediated Ligand Acceleration” J.Am. Chem. Soc. 119, 6189, 1997; L. Wang, H. Li, C. P. Wong, “Synthesisand Characterizations of Thermally Reworkable Epoxy Resins II”, J.Polym. Sci., Part A: Polym Chem., Vol. 38, No. 20, P. 3771 (2000). R.Mahidhara, “Comparing Chip-Scale Packaging to Direct Chip Attach,”ChipScale, May-June, 1999.

In view of the above problems associated with prior art methods offabricating underfilled microelectronic interconnect structures, thereis a need for developing a reworkable thermoset material so that partsattached to the thermoset adhesive can be reused after cleaning thethermoset residue. Moreover, a new and improved method of fabricatingmicroelectronic interconnect structures is needed which substantiallyeliminates the separate, time consuming, expensive underfill processeswhich are presently being carried out to fabricate the same.

SUMMARY OF THE INVENTION

One object of the present invention is to develop reworkable thermosetmaterials, which can be cleaved using a solvent. This rework processfacilitates thermoset residue removal and site cleaning so thatexpensive parts can be adhered to expensive substrates and, if needed,removed, cleaned and reused.

A further object of the present invention is to provide a method offabricating a reworkable microelectronic structure by spin coating aformulation containing a filler and a reworkable thermoset onto a waferto use in a modified wafer level underfill process.

In the present invention, b-stageable, reworkable thermoset materialshave been developed by introducing a cleavable acetal group [e.g.,—CH₂O—CHR¹—OCH₂—] or a ketal [(e.g., —CH₂O—CR¹R²—OCH₂—] [R¹ and R²groups can be varied as shown in FIGS. 2 b and 3 b] group into epoxyoligomers and/or co-oligomers. The b-stage T_(g) of the epoxy oligomerscan be varied by controlling the molecular weight of the oligomers or byusing co-monomers. The cured thermoset network cleaves in acidsolutions.

The invention also comprises a formulation of the reworkable epoxyoligomer, a filler and with or without solvent. The fully curedcomposite retains the reworkability character.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 a depicts the prior-art no-flow underfill process.

FIG. 1 b depicts the prior-art wafer level underfill process.

FIG. 1 c illustrates the IBM Wafer level underfill process.

FIG. 2 a is a schematic of the synthesis of an acetal based oligomer.

FIG. 2 b is a schematic of examples of acetal based oligomer.

FIG. 3 a is a schematic of the synthesis of an acetal based co-oligomer.

FIG. 3 b is a schematic of examples of an acetal based co-oligomer.

FIG. 4 a is a schematic of the epoxidation of an acetal based oligomer.

FIG. 4 b is a schematic of examples of epoxy compounds.

FIG. 5 is a schematic of examples of ketal based oligomer.

DETAILED DESCRIPTION OF THE INVENTION

The present invention concerns reworkable, waferlevel underfillmaterials and a new process. FIG. 1 c illustrates the IBM Wafer levelunderfill process. Acid-cleavable groups are introduced in epoxyoligomers. This invention provides chemical structures and proceduresfor the synthesis of said structures of acetal and ketal based epoxyoligomers.

FIG. 2 a depicts an example of the typical synthesis of an acetal basedoligomer wherein, for the purposes of the present invention, n is 6. Thereactants, the catalysts, the time of reaction, temperature andatmosphere are illustrative and can vary, with the essential requirementthat an acetal based bligomer be formed. Specific examples of suchacetal based oligomers are illustrated in FIG. 2 b.

FIG. 3 a depicts an example of a synthesis of an acetal basedco-oligomer, wherein for the purposes of the present invention, x is 3to 4; y is 3 to 4; and n is 3 to 4. In this case also, the reactants,the catalysts, the time of reaction, temperature and atmosphere canvary, with the essential requirement that a stable acetal basedco-oligomer be formed. Specific examples of such acetal basedco-oligomers are: given in FIG. 3 b.

The present invention also provides processes to accomplish cleaving ofthe cured thermoset network describing special solvents suitable forthis purpose. The invention describes further formulations of said epoxyoligomer with fillers and other additives such as solvents which can beused as waferlevel underfills. These formulations can be spin coatedonto wafers to exercise a modified waferlevel underfill process.

FIG. 4 a shows a typical example of an epoxidation of an acetal basedpolymer. Examples of epoxy compounds that can be used in accordance withthe present invention are depicted in FIG. 4 b.

Acids used to cleave the cured thermoset network are: suitable acidsinclude organic acids such as acetic acid, propionic acid, chloroaceticacid, benzoic acid and the like; sulfonic acids such as benzenesulfonicacid, p-toluene sulfonic acid, methane sulfonic acid and the like; andlewis acids such as boron trifluoride etherate, aluminum chloride,stannic chloride and the like.

Fillers used in the formulation are present in an amount between about5% and 75% by weight, preferably between about 30% and 65% by weight,and can be any of the powdered solids known in the art including ceramicparticles, such as alumina, silica, zinc oxide, BN, talc, titaniumoxide, metals such as Al, Ag, Cu and any nano-sized filler particletypes, including silica, TiO₂, clay,. etc.

The present invention also utilizes epoxy-containing monomers (distinctfrom the oligomers) which serve as reactive diluents. These reactivediluent monomers are different from those precursor monomers used toform the oligomers used in accordance with the present invention. Theepoxy-containing monomers which may be conveniently used as reactivediluents in accordance with the present invention include aromaticepoxies, aromatic diepoxies, aromatic cleavable epoxies,imide-containing epoxies, imide-containing diepoxies, imide-containingcleavable epoxies, aliphatic epoxies, aliphatic diepoxies, aliphaticcleavable epoxies, cycloaliphatic epoxies, cycloaliphatic diepoxies andcycloaliphatic cleavable epoxies. The function and method of using thesemonomeric epoxies is described in detail in Example 5 herein.

The following examples are given by way of illustration with theunderstanding that any of the individual compounds listed above canutilized for those disclosed in the methods presented.

EXAMPLE 1

This example discloses the preparation of an acetal oligomer:Cyclohexane dimethanol was reacted with tetrahydrobenzaldehyde andcyclohexyl methanol was used as an endcapper as shown in FIG. 2 a.

In a 250 ml three necked flask fitted with a mechanical stirrer, N₂inlet and outlet adapters, thermometer, Dean-Stark trap, and watercondenser, 26.07 g (0.1807 mol) of cyclohexane dimethanol, 22.71 g(0.2062 mol) of tetrahydrobenzaldehyde, and 5.79 g (0.0507 mol) ofcyclohexyl methanol were combined. 0.45 g of p-toluene sulfonic acid wasused as a catalyst. To the above mixture, 50 ml of dimethyl acetamideand 30 ml of toluene as an azeotropic solvent was added. The reactionmixture was heated to 140° C. in an oil bath for a 18 hrs. The waterevolved during the reaction was removed in a Dean-Stark trap to drivethe reaction to completion. After this reaction was complete, thetoluene was boiled off and the reaction was allowed to continue foranother 2 hrs. A viscous solution was obtained. The reaction mixture wascooled to room temperature, and the polymer was precipitated in a 75: 25water(basic):methanol mixture. A gummy polymer was obtained. The polymerwas redissolved in dichloromethane, washed twice with NaHCO₃ solution,and dried by passing through anhydrous sodium sulfate. The polymersolution was concentrated in a rotavap and then precipitated inmethanol. A gummy polymer was obtained and was dried in a vacuum oven at90° C. overnight. This resulted in a glassy polymer. The yield was ˜85%.The acetal oligomer was characterized by IR and NMR spectroscopictechniques. Although cyclohexane dimethanol was reacted withtetrahydrobenzaldehyde and cyclohexyl methanol used as an endcapper inaccordance with this example, other compounds disclosed in FIG. 2 a giveequally efficient results.

EXAMPLE 2

The schematic of the synthesis of a co-oligomer is shown in FIG. 3 a.The synthesis of the co-oligomer was carried out similar to that of theoligomer discussed above. However, cyclohexane carboxaldehyde was usedas a co-monomer. In the above scheme, the concentration oftetahydrobenzaldehyde to cyclohexane carboxaldehyde can be varied totune the cured material properties. The monomer and co-monomer can alsobe changed. In general, the dialcohol and aldehydes can be changed inacetal oligomer synthesis as shown in FIG. 3 b. The dialcohol can bealiphatic, aromatic or cyclic and aldehyde can be cyclic aliphatic oraromatic or any group illustrated in FIG. 3 b. Unsaturated aldehyde canbe used for epoxidation reaction. Aliphatic, aromatic or any other monoalcoholic group in FIG. 3 b can be used for contolling the molecularweight of the oligomer.

EXAMPLE 3

Epoxidation of the acetal oligomer: The epoxidation was carried out byadopting the literature procedure (J. Rudolph, K. Laxma Reddy, J. P.Chiang, and K. Barry Sharpless J. Am. Chem. Soc. 119, 6189, 1997) asshown in FIG. 4 a.

In a 250 ml three necked flask fitted with a mechanical stirrer, N₂inlet and outlet adapters, thermometer, and an addition funnel, 35 g(0.1494 mol) of acetal oligomer was dissolved in 200 ml ofdichloromethane. To this, 1.45 ml of pyridine (0.01793 ml) and 0.1862 g(0.00075 mol) of methyl trioxorhenium was added as a catalyst. Thereaction flask was cooled in an ice mixture. Exactly 25.6 ml (0.445 mol)of 50% hydrogen peroxide was added drop wise from an addition funnel sothat the temperature of the reaction mixture did not raise to more than5° C. After completed addition, the reaction was allowed to continue tostir at room temperature for 24 hrs. After the reaction, the aqueousphase was separated and discarded. The remaining H₂O₂ in the organicphase was decomposed to O₂ and H₂O by stirring with a catalytic amountof manganese dioxide (25 mg) until the color changed from yellow tocolorless. The polymer solution was washed twice with NaHCO₃ solutionand dried by passing through anhydrous sodium sulfate. The polymersolution was concentrated in a rotary evaporator and then precipitatedin methanol. A gummy polymer was obtained. It was dried in a vacuum ovenat 60° C. overnight. This resulted in a glassy polymer. The yield was˜55%. The polymer was analyzed by IR and NMR spectroscopic techniques.Epoxy Equivalent was 290 (theoretical 252).

EXAMPLE 4

Epoxidation of the acetal co-oligomer: The epoxidation was carried outusing the same procedure which was used in Example 3.

EXAMPLE 5

Epoxy Formulation and Spin Coating: The epoxy oligomer was mixed withhardener, catalyst, silica filler. Solvent was added to adjust theviscosity of the formulation for spin coating. Solvent content in theformulation can be reduced by adding mono or diepoxide reactive diluentsin place of solvent. Reactive diluents reduce solvent by first acting tomodify the viscosity during spin application (as would a solvent) butthen (unlike solvent) becomes immobilized in the network structure byreaction during final cure. Diepoxide reactive diluents perform theadditional function of modifying the network density and the finalproperties after curing. The reactive diluent can, for instance, bephenyl glycidyl ether (a mono epoxide) or bisphenol A diglycidyl ether(a diepoxide), 3,4-epoxycyclohexylmethyl 3,4-epoxycyclohexanecarboxylate(another diepoxide), or a cleavable diepoxide of a type described in theprior art by Buchwalter et. at. Inclusion of cleavable diepoxide as anexample of a reactive diluent is a matter of semantics because in thiscase this small molecule performs three functions: It lowers theviscosity because it is small relative to the large and viscousoligomers, it increases the network density because it is multifuncionaland short, and additionally increases the density of acid cleavablegroups in the network. The formulation was mixed well, degassed and keptin a freezer at −40° C. To use it was thawed to room temperature andspin coated on a wafer under conditions leading to the required filmthickness, i.e., so that the underfill film covers the solder bumps to adesired extent. The formulation was spin coated onto a silicon waferfollowed by b-stage curing to get a tack free surface and the desiredb-stage Tg, flow, and storage shelf life properties. The wafer is thendiced to form discrete chips. The chips can optionally be briefly heatedabove the Tg on a hot surface to allow the underfill to form a slightconvex shape over the chip area which gives certain advantages duringjoining by reducing the entrapment of air. It also improves visualtransparency near the edges where alignment marks can be located. Thechip can then be placed, aligned, and joined to the substrate.

EXAMPLE 6

Proof of Reworkability: The epoxy oligomer was mixed with hardener andcatalyst. The mixture was then spotted onto glass slides forming dots of˜1 cm diameter and cured in an oven at 150° C. for 2 hrs. This treatmentfully cured the material of said spots into a hard, crosslinked tackfree solid, as expected for a thermoset. The cured epoxy was dissolvedin a methane sulfonic acid/trifluroethanol (3.2 g/100 ml) mixture at 80°C. The cured samples dissolved within 4-6 minutes, demonstrating thecleavability and thus the reworkability.

The molecular weight of the oligomers and co-oligomers as used in thepresent invention is around 2000.

Applicants have determined that the compositions embodied within thescope of the present invention may be blended with thermoplasticpolymers to increase the impact performance of the thermoset.Thermoplastics such as polyethylene, polypropylene, polybutene,polysulfone, polycarbonate, polyesters, etc. and any of the other wellknown thermoplastic polymers that modify impact properties may be used.U.S. Pat. No. 6,225,373 to Hedrick, the contents of which areincorporated by reference herein discloses modifying an epoxy systemwith a thermoplastic polymer.

While we have described our preferred embodiments of our invention, itwill be understood that those skilled in the art, both now and in thefuture, may make various improvements and enhancements which fall withinthe scope of the claims which follow. These claims should be construedto maintain the proper protection for the invention first disclosed.

1. A reworkable thermoset which can be b-staged into a tack free stateand which softens considerably during heating before final curecomprising: an epoxy-containing oligomer having the formula:

wherein X is the same or different, and is selected from the groupconsisting of:

at least one R¹ must be a cleavable epoxy, and if more than one, may bethe same or different, and said R¹ being selected from the groupconsisting of:

any R¹ which is not an epoxy, may be a precursor to said epoxies and maybe the same or different, said R¹ is selected from the group consistingof:

Q is selected from the group consisting of:

and n is 1 to 6 said epoxy groups, in said epoxy-containing oligomer,after said epoxy-containing oligomer is cured, being cleavable using anacid selected from the group consisting of an organic acid and a Lewisacid.
 2. The thermoset defined in claim 1 comprising a mixture of saidoligomers having different X, R¹ and Q groups.
 3. The thermoset definedin claim 1 in admixture with monomers selected from the group consistingof aromatic epoxies, aromatic diepoxies and aromatic cleavable epoxies,aliphatic epoxies, aliphatic diepoxies and aliphatic cleavable epoxies,imide-containing epoxies, imide-containing diepoxies andimide-containing cleavable epoxies, cycloaliphatic epoxies,cycloaliphatic diepoxies and cycloaliphatic cleavable epoxies.
 4. Thethermoset defined in claim 1 wherein a moiety which is cleavable is anacetal group.
 5. The thermoset defined in claim 1 wherein a moiety whichis cleavable is a ketal group.
 6. The thermoset defined in claim 1 whichcontains chemically cleavable groups selected from the group consistingof acetal and ketal groups.
 7. The thermoset defined in claim 3 whereineach said monomer contains chemically cleavable groups selected from thegroup consisting of acetal and ketal groups.
 8. The thermoset defined inclaim 3 wherein only a portion of said monomers contains chemicallycleavable groups selected from the group consisting of acetal and ketalgroups.
 9. The thermoset defined in claim 1 in admixture with betweenabout 5% and 40% by weight of a thermoplastic.
 10. A formulationcomprising the thermoset defined in claim 1 in admixture with a betweenabout 5% and 75% by weight of a filler.
 11. A formulation comprising thethermoset defined in claim 9 in admixture with between about 5% and 75%by weight of a filler.
 12. A formulation comprising the thermosetdefined in claim 1 in admixture with between about 0.2% and 5% by weightof a flux.
 13. A formulation comprising the thermoset defined in claim 1in admixture with a plurality of different fluxes.
 14. A formulationcomprising the thermoset defined in claim 1 in admixture with ahardener.
 15. A formulation comprising the thermoset defined in claim 1in admixture with a plurality of different hardeners.
 16. A formulationcomprising the thermoset defined in claim 1 in admixture with acatalyst.
 17. A formulation comprising the thermoset defined in claim 1in admixture with a plurality of different catalysts.
 18. Theformulation defined in claim 14 which also contains a catalyst.
 19. Theformulation defined in claim 18 which is an adhesive that is not CTEmatched.
 20. The formulation defined in claim 18 which also contains afiller.
 21. The formulation defined in claim 20 which is a CTE matchedhigh impact composition useful as an adhesive.
 22. The formulationdefined in claim 18 which also contains a flux.
 23. The formulationdefined in claim 22 which also contains a catalyst.
 24. A process formaking the reworkable thermoset defined in claim 1 comprising reacting amixture of difunctional and monofunctional primary alcohols withaldehydes having unsaturated groups and then reacting the resultingreaction products using an epoxidation reaction.
 25. The method definedin claim 24 wherein the ratio of difunctional and monofunctionalalcohols is selected to result in an oligomer molecular weight of about2000.
 26. The method defined in claim 25 wherein the difunctionalalcohol is cyclohexanedimethanol.
 27. The method defined in claim 25wherein said monofunctional alcohol is cyclohexylmethanol.
 28. Themethod defined in claim 25 wherein said difunctional alcohol iscyclohexanedimethanol.
 29. The method defined in claim 25 wherein saidaldehyde is tetrahydrobenzaldehyde.
 30. The method defined in claim 25wherein said aldehyde is a mixture of tetrahydrobenzaldehyde andcyclohexane carboxaldehyde.
 31. The method defined in claim 30 whereinsaid mixture of tetrahydrobenzaldehyde and cyclohexane carboxaldehyde isadjusted to control the amount of crosslinkable epoxy groups in saidoligomer.
 32. A structure comprising the formulation defined in claim 21in contact with a substrate.
 33. The structure defined in claim 32wherein said substrate is a circuitized and bumped wafer.
 34. Thestructure defined in claim 32 wherein said substrate is an opticalcomponent selected from the group consisting of a laser diode or a photodiode.
 35. A method for applying the formulation defined in claim 1 to asubstrate wherein said formulation is applied by means selected from thegroup consisting of spin coating or screening.
 36. A structure obtainedby b-staging to a tack free surface the formulation defined in claim 1on a circuitized and bumped wafer.
 37. The b-staged chip structuredefined in claim 36 obtained by dicing said wafer.
 38. A reworkableassembly comprising the structure defined in claim 36 joined to anotherelectronic element.
 39. A reworkable package obtained by joining saidchip defined in claim 37 to an electronic substrate.
 40. The reworkablepackage defined in claim 39 wherein said substrate is an organiclaminate.
 41. A process for joining the reworkable package defined inclaim 30 comprising: aligning bumps on a chip to corresponding metalpads on said substrate; heating the aligned assembly, applying pressureto push said b-staged formulation out of the way of said bumps so thatmetal to metal contact is made between said bumps and said pads; heatingthe package until a soldered contact between the metal of the bumps andthe metal of said pads is achieved.
 42. The structure resulting from themethod defined in claim
 41. 43. The structure defined in claim 42 inwhich said formulation which has been pushed out of the way of saidbumps forms a fillet.
 44. The structure defined in claim 42 in whichsaid formulation is substantially cured.
 45. A process wherein saidjoined assembly of claim 36 is reworked by heating and separating joinedparts.
 46. A process in which residue of said thermoset on saidseparated parts defined in claim 45 is substantially removed byapplication of a chemical agent.
 47. The process defined in claim 46wherein said chemical agent is an acid.
 48. The process defined in claim46 wherein said chemical agent is a mixture of acids.
 49. The processdefined in claim 46 wherein said chemical agent is an acid or mixture ofacids in a solvent.
 50. The process defined in claim 47 wherein saidchemical agent is an acid selected from the group consisting of aceticacid, phosphoric acid, p-toluene sulfonic acid and methane sulfonicacid.
 51. The process defined in claim 4 wherein said chemical agent isan methane sulfonic acid.
 52. The process defined in claim 49 whereinsaid solvent is selected from the group consisting of water, ethanol,y-butyrolactone, N-methylpyrrolidone, xylene, benzyl alcohol andtrifluoromethanol.
 53. The process defined in claim 52 wherein saidsolvent is trifluoromethanol.
 54. The formulation defined in claim 10comprising the thermoset defined in claim 1 in admixture with a betweenabout 30% and 65% by weight of a filler.
 55. The formulation comprisingthe thermoset defined in claim 12 in admixture with between about 1.5%and 2.5% by weight of a flux.
 56. The reworkable thermoset defined inclaim 1 wherein said organic acid is selected from the group consistingof acetic acid, propionic acid, chloroacetic acid, benzoic acid,benzenesulfonic acid, p-toluene sulfonic acid, methane sulfonic acid.57. The reworkable thermoset defined in claim 1 wherein said Lewis acidis selected from the group consisting of boron trifluoride etherate,aluminum chloride, stannic chloride.
 58. The formulation defined inclaim 10 wherein said filler is selected from the group consisting ofceramic particles, metals and nano particles with good opticalproperties.
 59. The formulation defined in claim 58 wherein said filleris a ceramic particle selected from the group consisting of SiO₂, BN,alumina, TiO₂ and talc.
 60. The formulation defined in claim 58 whereinsaid filler is a metal selected from the group consisting of Al, Ag andCu.
 61. The formulation defined in claim 58 wherein said filler is anano particle with good optical properties selected from the groupconsisting of SiO₂, clay, and TiO₂.